Have you ever walked out into your backyard and looked for food? Most people haven't, but I do it all the time. I have a garden like many, but what I'm talking about is wild edibles. Food is all around you, even in a fairly urban area, such as where I live in Poughkeepsie New York.
The easiest thing to find is Dandelions, the entire plant is edible. (Although I must admit, they're a little bitter for my taste) Try blanching the leaves in boiling water for a minute or two, helps a lot. Also one thing I've been looking to try is making a tea from the flowers. I've also heard that it's a spectacular addition to Mead, so I'll be trying that next batch.
One of my other go to plants is wild onions. I'm sure you've noticed them in early spring, they look like tall grass and sprout up in early may. Super easy to gather and easy to spot, they are a great wild vegetable and would provide at least some calories in a survival situation. The ones I gathered this spring made an excellent onion soup. I take all my onions, cleaned thoroughly and chopped off at the point they begin to turn green, give them a rough chop and sauteed them with butter and garlic. Add beef or pork stock, season to taste and simmer for two hours. The onions are a little chewier than the store bought stuff, but the flavor is ten times better.
If you can find them, grapes are the best. They grow all over around here, and I'm fortunate enough to have more growing in my backyard than I could ever use. An excellent survival food, all sugar, that's if you can get past all the seeds. They're usually ready early august. Mine will be relegated to making wine mostly, very excited to make something that every ingredient came from 100 meters from my bed.